Method for producing composition containing low molecular weight polytetrafluoroethylene

ABSTRACT

A method for producing a composition containing low molecular weight polytetrafluoroethylene, the method including: (I) irradiating a composition containing high molecular weight polytetrafluoroethylene with ionizing radiation to obtain a composition containing low molecular weight polytetrafluoroethylene having a melt viscosity at 380° C. in the range of 1.0×102 to 7.0×105 Pa·s; and (II) carrying out at least one selected from the group consisting of washing treatment, steam treatment and decompression treatment on the composition containing low molecular weight polytetrafluoroethylene.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a National Stage of International Application No.PCT/JP2019/004048 filed Feb. 5, 2019, claiming priority based onJapanese Patent Application No. 2018-020426, filed Feb. 7, 2018.

TECHNICAL FIELD

The present invention relates to a method for producing a compositioncontaining low molecular weight polytetrafluoroethylene.

BACKGROUND ART

Since low molecular weight polytetrafluoroethylene (sometimes referredto as “low molecular weight PTFE” hereinafter) not only has excellentchemical stability and extremely low surface energy but also is unlikelyto suffer fibrillation, it has been used for producing plastics, inks,cosmetics, coating materials, greases, etc. as an additive for enhancinglubricity or texture of a coating surface (e.g., Patent Literature 1).One of the methods for producing a composition containing such lowmolecular weight PTFE is a method of irradiating high molecular weightpolytetrafluoroethylene (sometimes referred to as “PTFE” hereinafter)with ionizing radiation (e.g., Patent Literature 2).

CITATION LIST Patent Literature

-   Patent Literature 1: Japanese Patent Laid-Open No. 10-147617-   Patent Literature 2: Japanese Patent Publication No. 47-19609

SUMMARY OF INVENTION Technical Problem

In such a composition containing low molecular weight PTFE as in PatentLiterature 2, a low molecular weight fluorine-containing compound(sometimes referred to as “PFC” hereinafter), such as perfluorooctanoicacid (sometimes referred to as “PFOA” hereinafter) or its salt, can becontained. As a result of earnest studies by the present inventor, ithas been found that PFC typified by such PFOA as above can be producedby irradiation with ionizing radiation.

It is an object of the present invention to obtain a compositioncontaining low molecular weight PTFE, which has been reduced in acontent of PFC.

Solution to Problem

According to a first aspect of the present invention, provided is amethod for producing a composition containing low molecular weight PTFE,comprising:

(I) irradiating a composition containing PTFE with ionizing radiation toobtain a composition containing low molecular weight PTFE having a meltviscosity at 380° C. in the range of 1.0×10² to 7.0×10⁵ Pa·s; and

(II) carrying out at least one selected from the group consisting ofwashing treatment, steam treatment and decompression treatment on thecomposition containing low molecular weight PTFE.

Advantageous Effects of Invention

According to the present invention, a method suitable for producing acomposition containing low molecular weight PTFE, which has been reducedin a content of PFC, can be obtained.

DESCRIPTION OF EMBODIMENTS

The method for producing a composition containing low molecular weightPTFE of the present invention comprises:

(I) irradiating a composition containing PTFE with ionizing radiation toobtain a composition containing low molecular weight PTFE having a meltviscosity at 380° C. in the range of 1.0×10² to 7.0×10⁵ Pa·s (sometimesreferred to as “step (I)” hereinafter); and

(II) carrying out at least one selected from the group consisting ofwashing treatment, steam treatment and decompression treatment on thecomposition containing low molecular weight PTFE (sometimes referred toas “step (II)” hereinafter).

The step (I) will be described hereinafter.

The PTFE can be one obtained by a polymerization method that can beusually carried out, such as emulsion polymerization or suspensionpolymerization.

The PTFE may be one having, as a constituent unit derived from a monomerin a molecular structure, only a constituent unit derived fromtetrafluoroethylene (TFE), or a constituent unit derived from a monomerhaving a structure other than TFE and a constituent unit derived fromTFE (sometimes referred to as “modified PTFE” hereinafter).

The monomer having a structure other than TFE is not limited as long asit is copolymerizable with TFE, and a monomer usually used (e.g.,perfluoroolefin such as hexafluoropropylene [HFP]; chlorofluoroolefinsuch as chlorotrifluoroethylene [CTFE]; hydrogen atom-containingfluoroolefin such as trifluoroethylene or vinylidene fluoride [VDF];perfluorovinyl ether; perfluoroalkylethylene; ethylene) can be used.Only one of the monomers having a structure other than TFE may be used,or a plurality of them may be used.

Examples of the perfluorovinyl ether include, but not limited to, anunsaturated perfluoro compound represented by the following generalformula (1):CF₂═CF—ORf  (1)wherein Rf represents a perfluoro organic group. In the presentspecification, the “perfluoro organic group” means an organic group inwhich hydrogen atoms bonded to carbon atoms are all substituted byfluorine atoms. Examples of the perfluoro organic group include aperfluoroalkyl group and a perfluoro(alkoxyalkyl) group. The perfluoroorganic group may have an oxygen atom that forms an ether linkage.

In one embodiment, the perfluorovinyl ether is, for example, aperfluoroalkyl vinyl ether (PAVE), wherein Rf is a perfluoroalkyl grouphaving 1 to 10 carbon atoms (preferably 1 to 5 carbon atoms) in theabove general formula (1).

Examples of the perfluoroalkyl group in the PAVE include aperfluoromethyl group, a perfluoroethyl group, a perfluoropropyl group,a perfluorobutyl group, a perfluoropentyl group and a perfluorohexylgroup. A preferred perfluoroalkyl group is, for example, aperfluoropropyl group.

Preferred PAVE is perfluoropropyl vinyl ether (PPVE) in which theperfluoroalkyl group is a perfluoropropyl group.

In another embodiment, the perfluorovinyl ether is, for example, in thegeneral formula (1), one in which Rf is a perfluoro(alkoxyalkyl) grouphaving 4 to 9 carbon atoms;

one in which Rf is a group represented by the following formula:

wherein m is 0 or an integer of 1 to 4; or

one in which Rf is a group represented by the following formula:

wherein n is an integer of 1 to 4.

Examples of the perfluoroalkylethylene include, but not limited to,(perfluorobutyl)ethylene (PFBE), (perfluorohexyl)ethylene and(perfluorooctyl)ethylene.

The monomer having a structure other than TFE is preferably at least oneselected from the group consisting of HFP, CTFE, VDF, PPVE, PFBE andethylene, and is more preferably at least one selected from the groupconsisting of HFP and CTFE.

In the modified PTFE, the constituent unit derived from the monomerhaving a structure other than TFE is preferably contained in an amountin the range of 0.001 to 1 mass %. In the modified PTFE, the constituentunit derived from the monomer having a structure other than TFE is morepreferably contained in an amount of 0.01 mass % or more. In themodified PTFE, the constituent unit derived from the monomer having astructure other than TFE is more preferably contained in an amount of0.5 mass % or less, and is still more preferably contained in an amountof 0.1 mass % or less. The content of the constituent unit derived fromthe monomer having a structure other than TFE can be determined by aknown method such as Fourier transform infrared spectroscopy (FT-IR).

A standard specific gravity (SSG) of the PTFE is preferably in the rangeof 2.130 to 2.230. The SSG is a value measured in accordance with ASTM D4894 when the PTFE is obtained by suspension polymerization, and is avalue measured in accordance with ASTM D 4895 when the PTFE is obtainedby emulsion polymerization. SSG or a melt viscosity can be usually usedas an indication of a molecular weight of PTFE. PTFE having such SSG asabove has an extremely high melt viscosity, and it is difficult toaccurately measure the melt viscosity. Accordingly, in PTFE having suchSSG as above, the SSG is usually used as an indication of the molecularweight.

A melting point of the PTFE is preferably in the range of 324° C. to336° C. The melting point is a value measured by using, for example, adifferential scanning calorimeter (DSC). Specifically, the melting pointcan be a minimum point of heat of fusion, which is obtained by using aDSC having been subjected to temperature calibration in advance usingindium and lead as standard samples and increasing the temperature at10° C./min in the temperature region of 250° C. to 380° C. in an airstream at 200 ml/min.

The composition containing PTFE is preferably substantially composed ofPTFE. The expression “substantially composed of PTFE” means that PTFEcan be contained in an amount of 90 mass parts or more, specifically inan amount of 95 mass parts or more, based on 100 mass parts of thecomposition. The upper limit of the content of PTFE based on 100 massparts of the composition is not limited, but it can be, for example, 100mass parts or less, specifically 98 mass parts or less.

The composition containing PTFE may be one containing PFOA or its salt,or may be one substantially free from PFOA or its salt. Here, theexpression “substantially free from” means that the amount of PFOA, interms of mass, based on PTFE is less than 25 ppb, preferably 15 ppb orless, more preferably 5 ppb or less, particularly preferably less thanthe detection limit. The expression “less than the detection limit”means, for example, less than 5 ppb. In the present specification, theabove content indicates a total amount of PFOA and its salt, unlessotherwise noted.

The composition containing PTFE is preferably substantially free fromPFOA or its salt. Here, the expression “substantially free from” has thesame meaning as above. Even when the composition containing PTFE andsubstantially free from PFOA or its salt is used as in the presentembodiment, PFOA or its salt can be formed by irradiating the PTFE withionizing radiation in the step (I).

Examples of the salt of PFOA include an ammonium salt, a sodium salt anda potassium salt of PFOA.

The content of the PFOA and its salt can be measured by using liquidchromatography. More specifically, the PFOA and its salt are extractedfrom the composition containing PTFE using a solvent, and the amount ofthe PFOA and its salt contained in the solvent after extraction can bemeasured by using liquid chromatography.

As the composition containing PTFE, one containing PFC may be used, orone substantially free from PFC may be used. Here, the expression“substantially free from” means that the amount of PFC, in terms ofmass, based on PTFE is less than 25 ppb, preferably 15 ppb or less, morepreferably 5 ppb or less, particularly preferably less than thedetection limit. The expression “less than the detection limit” means,for example, less than 5 ppb.

The composition containing PTFE is preferably substantially free fromPFC. Even when the composition containing PTFE and substantially freefrom PFC is used as in the present embodiment, PFC can be formed byirradiating the PTFE with ionizing radiation in the step (I). Here, theexpression “substantially free from” has the same meaning as above. Inthe present specification, when PFCs of a plurality of structures ispresent, the above content indicates a total amount of PFCs of aplurality of structures, unless otherwise noted.

The content of the PFC can be measured by using, for example, liquidchromatography. More specifically, from the composition containing PTFE,the PFC is extracted using a solvent, and the amount of the PFCcontained in the solvent after extraction can be measured by usingliquid chromatography.

In the present specification, PFC means a low molecular weightfluorine-containing compound such as PFOA or its salt, specificallymeans a low molecular weight fluorine-containing compound having afunctional group or its derivative, and more specifically means an acidhaving a fluorine-containing carbon chain or its derivative. Thefluorine-containing carbon chain indicates a carbon chain in which oneor more of hydrogen atoms bonded to carbon atoms are substituted byfluorine atoms. Examples of the acid include carboxylic acid andsulfonic acid. Examples of the derivative include salts. Examples of thesalt include an ammonium salt, a sodium salt and a potassium salt.

Examples of the PFC include an acid having a fluorine-containing carbonchain having 6 to 14 carbon atoms and its derivative.

Examples of the shape of the composition containing PTFE include, butnot limited to, a powder, a molded body, a scrap, a fragment that can beproduced during formation of a molded body, a cutting chip producedduring cutting of a molded body, and a preform.

In one embodiment, the shape of the composition containing PTFE ispowder. The present embodiment is advantageous from the viewpoint thatthe resin component containing PTFE can be uniformly irradiated withionizing radiation. According to the present embodiment, a powderycomposition containing low molecular weight PTFE can be easily obtained.

In one embodiment, the composition containing PTFE is a molded body. Inthe present embodiment, the composition containing PTFE can be onehaving been subjected to a molding step of heating the compositioncontaining PTFE to a primary melting point or higher. That is to say,the production method of the present invention can further include,prior to the step (I), a step of heating the composition containing PTFEto a primary melting point of PTFE or higher to form a molded body ofthe composition containing PTFE.

The primary melting point can be measured by using DSC. Specifically,the primary melting point is a maximum peak temperature of anendothermic curve appearing on a crystal melting curve, and theendothermic curve is obtained by heating uncalcined PTFE under theconditions of a temperature-increasing rate of 10° C./min. The primarymelting point can be usually a temperature of 320° C. or higher. Theuncalcined PTFE indicates PTFE free from heat history of being heated toa temperature of the primary melting point or higher.

The shape of the molded body is not limited. Examples of the shape ofthe molded body include tape-like, sheet-like, rod-like, tubular, andfibrous shapes.

A specific gravity of the molded body is preferably 1.0 g/cm³ or more,more preferably 1.5 g/cm³ or more, and is preferably 2.5 g/cm³ or less.The specific gravity can be measured by an underwater replacementmethod.

In one embodiment, the composition containing PTFE is a scrap, afragment that can be produced during formation of a molded body, or acutting chip produced during cutting of a molded body.

In one embodiment, the composition containing PTFE is a preform. Here,the preform refers to a molded body that is obtained by compressingpowdery PTFE and has not been subjected to a calcining step (has notbeen heated to a temperature of the primary melting point or higher).

Irradiation with ionizing radiation can be carried out by aconventionally known method and under the conventionally knownconditions. By the irradiation with ionizing radiation, the PTFEcontained in the composition containing PTFE is decreased in molecularweight and becomes low molecular weight PTFE. By the irradiation withionizing radiation, PFC such as PFOA or its salt can be produced.

Examples of the ionizing radiation include electron ray, γ-ray, X-ray,neutron beam and high energy ion. The ionizing radiation is preferablyelectron ray or γ-ray.

The irradiation with ionizing radiation is not limited, but can becarried out in, for example, air, a hydrocarbon-based gas, water, or asolvent.

In one embodiment, the irradiation with ionizing radiation can becarried out in air. The present embodiment is preferable from theviewpoint of reduction of cost.

An irradiation dose of the ionizing radiation is preferably in the rangeof 1 to 2,500 kGy. The irradiation dose of the ionizing radiation ismore preferably 1,000 kGy or less, still more preferably 750 kGy orless. The irradiation dose of the ionizing radiation is more preferably10 kGy or more, particularly preferably 50 kGy or more.

An irradiation temperature of the ionizing radiation is, for example, 0°C. or higher. The upper limit of the irradiation temperature of theionizing radiation is not limited as long as it is a melting point ofPTFE or lower. From the viewpoint of suppression of crosslinking of PTFEmolecular chain, the irradiation temperature of the ionizing radiationis preferably 320° C. or lower, more preferably 300° C. or lower, stillmore preferably 260° C. or lower. From the viewpoint of reduction ofproduction cost, irradiation with ionizing radiation can be carried outat 15° C. (e.g., 19° C.) to 60° C.

The resin component contained in the composition obtained in the step(I) is preferably substantially composed of low molecular weight PTFE.The expression “substantially composed of low molecular weight PTFE”means that the low molecular weight PTFE can be contained in an amountof, for example, 90 mass parts or more, specifically 95 mass parts ormore, based on 100 mass parts of the composition. The upper limit of thelow molecular weight PTFE based on 100 mass parts of the composition isnot limited, but it can be, for example, 100 mass parts or less,specifically 98 mass parts or less.

The shape of the composition obtained in the step (I) is not limited.

In one embodiment, the shape of the composition obtained in the step (I)is powder. This composition is advantageous from the viewpoint of easeof treatment in or after the step (II).

In the above embodiment, an average particle size of the low molecularweight PTFE in the composition obtained in the step (I) is preferably1,000 μm or less, more preferably 300 μm or less, still more preferably100 μm or less. The lower limit of the average particle size of the lowmolecular weight PTFE is not limited, but it is, for example, more than1 μm. Since the low molecular weight PTFE has such an average particlesize, the diameters of particles contained in the composition can becomerelatively small.

The average particle size of the low molecular weight PTFE can bemeasured by using a laser diffraction particle size distributionmeasurement apparatus. Specifically, the average particle size is takento be equal to a particle size corresponding to 50% (based on volume) ofparticle size distribution integration obtained by carrying outmeasurement at a dispersion pressure of 1.0 bar without using a cascade.The laser diffraction particle size distribution measurement apparatusmay be, for example, laser diffraction particle size distributionmeasurement apparatus manufactured by JEOL Ltd. (trade name: HELOS &RODOS).

A melt viscosity of the low molecular weight PTFE at 380° C. is in therange of 1.0×10² to 7.0×10⁵ Pa·s. A more preferred lower limit of themelt viscosity is 1.5×10³ Pa·s or more. A more preferred upper limit ofthe melt viscosity is 3.0×10⁵ Pa·s or less. In the present invention,the “low molecular weight PTFE” means PTFE having a melt viscosity inthe above range. In the case of low molecular weight PTFE having such amelt viscosity as above, it is difficult to form a molded body for usein the measurement of the aforesaid standard specific gravity (SSG), andit is difficult to measure accurate SSG of the low molecular weightPTFE. Accordingly, a melt viscosity is usually used as an indication ofa molecular weight of low molecular weight PTFE.

In accordance with ASTM D 1238, the melt viscosity can be measured using2 g of a sample having been heated in advance at the measuringtemperature (380° C.) for 5 minutes and using a flow tester(manufactured by Shimadzu Corporation) and a die of 2ϕ-8L whilemaintaining the sample at the above temperature under a load of 0.7 MPa.

The low molecular weight PTFE can be non-fibrillatable.

A melting point of the low molecular weight PTFE is preferably in therange of 324° C. to 336° C. This melting point can be measured using amethod described above as the method for measuring a melting point ofPTFE.

The step (II) will be described hereinafter.

The step (II) is a step in which at least one selected from the groupconsisting of washing treatment, steam treatment and decompressiontreatment is carried out on the composition containing low molecularweight PTFE obtained in the step (I). By carrying out the step (II), acomposition containing low molecular weight PTFE and having a lowcontent of PFC can be obtained.

In other words, the step (II) is a step in which PFC is removed from thecomposition containing low molecular weight PTFE obtained in the step(I) to form a composition containing low molecular weight PTFE andhaving a low content of PFC.

Although the content of PFC contained in the composition containing lowmolecular weight PTFE obtained in the step (I) is not limited, it is,for example, 25 mass ppb or more based on the low molecular weight PTFE.

A content of the PFOA or its salt contained in the compositioncontaining low molecular weight PTFE obtained in the step (I) is notlimited, but it is, for example, 25 mass ppb or more based on the lowmolecular weight PTFE.

The at least one treatment selected from the group consisting of washingtreatment, steam treatment and decompression treatment may be carriedout only once, or may be carried out a plurality of times. When theseare carried out a plurality of times, the same method may be repeatedlycarried out, or different methods may be carried out in combination.

The washing treatment can be carried out using a solution containing atleast one selected from the group consisting of water and an organicsolvent (sometimes referred to as a “washing solution” hereinafter).

It is preferable to use a polar solvent as the organic solvent. Examplesof the polar solvent include alcohols, such as methanol, ethanol, andisopropyl alcohol (IPA); ketones, such as acetone, methyl ethyl ketone,methyl isobutyl ketone (MIBK), and diisobutyl ketone (DIBK); nitriles,such as acetonitrile; N,N-dimethylformamide, and dimethyl sulfoxide.

It is preferable to use an alkaline solution as the washing solution,and it is more preferable to use a solution of ammonia, sodium hydroxideor the like. From the viewpoint of obtaining low molecular weight PTFEhaving a low content of a metallic atom, a solution of ammonia ispreferably used.

The washing treatment may be carried out only once, or may be carriedout a plurality of times. When the washing treatment is carried out aplurality of times, the washing solutions may be the same as oneanother, or may be different from one another. The washing solutions maybe those of the same concentration, or may be those of differentconcentrations.

The washing treatment can be carried out by applying at least oneselected from the group consisting of stirring, heating and ultrasonicwaves to the composition immersed in the washing solution.

A method for applying stirring is not limited, and a method that can beusually used, such as a method of using a stirrer or a stirring blade,can be used.

A method for applying ultrasonic waves is not limited, and a method thatcan be usually used can be used.

The washing treatment can be carried out under pressure, at normalpressure or under reduced pressure. From the viewpoint of suppression ofproduction cost, the washing treatment is preferably carried out atnormal pressure.

In a preferred embodiment, the washing treatment is carried out by usingwater, an alkaline aqueous solution or a polar solvent as the washingsolution and applying at least one selected from the group consisting ofstirring, heating and ultrasonic waves to the composition immersed inthe washing solution.

In a preferred embodiment, water washing treatment is further carriedout after the washing treatment. The water washing treatment ispreferably carried out until pH of wash water becomes 6.5 to 8.0.

In a more preferred embodiment, drying treatment is carried out afterthe washing treatment.

A method for carrying out the drying treatment is not limited, but thedrying treatment can be carried out using, for example, a box typedryer, a hot air circulation type dryer, a vacuum dryer, a spray dryer,a vibration dryer, a fluidized bed dryer, or a fixed bed dryer. Thedrying treatment can be specifically carried out using a box type dryer,a hot air circulation type dryer or the like.

The drying treatment can be carried out in the temperature range of, forexample, 100° C. to 260° C. From the viewpoints of productivity andsafety, the drying treatment is preferably carried out in thetemperature range of 100° C. to 140° C.

The drying treatment can be carried out in order to remove water, asolvent or the like used for the washing. The time for carrying out thedrying treatment can be set to, for example, 5 to 24 hours.

By carrying out the drying treatment, the content of PFC contained inthe composition containing low molecular weight PTFE obtained by theproduction method of the present invention can be further reduced.

By carrying out the drying treatment, the content of PFOA or its saltcontained in the composition containing low molecular weight PTFEobtained by the production method of the present invention can befurther reduced.

The steam treatment can be carried out by bringing a treatment solutionin the form of steam and the composition containing low molecular weightPTFE provided to the step (II) into contact with each other.

The treatment solution may be water, an alkaline aqueous solution, or apolar solvent.

The steam treatment is preferably carried out using steam of 90° C. to160° C., and is more preferably carried out using steam of 95° C. to120° C.

The steam treatment can be carried out using water, an alkaline solutionor a polar solvent as the treatment solution in the form of steam at atemperature in the range of 90° C. to 160° C.

The steam treatment is preferably carried out using an alkaline solutionhaving been heated to 90° C. to 160° C., and is more preferably carriedout using an alkaline solution having been heated to 95° C. to 120° C.

The steam treatment is preferably carried out under pressure. The steamtreatment is more preferably carried out at a pressure in the range of0.1 to 0.2 MPa, and is still more preferably carried out at a pressurein the range of 0.1 to 0.15 MPa.

In a preferred embodiment, the steam treatment can be carried out at atemperature in the range of 90° C. to 160° C. though the temperaturevaries depending on the treatment solution.

The steam treatment can be carried out by bringing the treatmentsolution in the form of steam and the composition containing lowmolecular weight PTFE provided to the step (II) into contact with eachother for, for example, 10 minutes to 1 hour.

In a more preferred embodiment, drying treatment is carried out afterthe steam treatment. The drying treatment can be carried out in the samemanner as described above.

By carrying out the drying treatment, the content of PFC contained inthe composition containing low molecular weight PTFE obtained by theproduction method of the present invention can be further reduced.

By carrying out the drying treatment, the content of PFOA or its saltcontained in the composition containing low molecular weight PTFEobtained by the production method of the present invention can befurther reduced.

The decompression treatment can be carried out at a gauge pressure of−0.01 to −0.1 MPa, specifically −0.05 to −0.1 MPa.

The time for carrying out the decompression treatment is not limited,but it is in the range of, for example, several minutes to 10 days.

A method for carrying out the decompression treatment is not limited,but the decomposition treatment can be carried out by using, forexample, a vacuum dryer, an accumulator, or a double cone vacuumstirring machine.

The step (II) can further include grinding treatment. Specifically, thestep (II) can be a step in which at least one selected from the groupconsisting of washing treatment, steam treatment and decompressiontreatment is carried out on the composition containing low molecularweight PTFE obtained in the step (I), and grinding treatment is carriedout. The washing treatment, the steam treatment and the decompressiontreatment are as described above.

In one embodiment, the step (II) can be a step in which at least oneselected from the group consisting of washing treatment, steam treatmentand decompression treatment is carried out on the composition containinglow molecular weight PTFE obtained in the step (I), and thereafter,grinding treatment is carried out.

In one embodiment, the step (II) can be a step in which grindingtreatment is carried out on the composition containing low molecularweight PTFE obtained in the step (I), and thereafter, at least oneselected from the group consisting of washing treatment, steam treatmentand decompression treatment is carried out. In the above step, aplurality of treatment may be carried out in combination.

In a preferred embodiment, in the step (II), grinding treatment iscarried out on the composition containing low molecular weight PTFEobtained in the step (I), and thereafter, at least one selected from thegroup consisting of washing treatment, steam treatment and decompressiontreatment is carried out on the grinding-treated low molecular weightPTFE composition. According to the present embodiment, removal of PFCtypified by PFOA or its salt can be more effective.

A method for the grinding treatment is not limited, but it is, forexample, a method of grinding using a grinder. Examples of the grinderinclude impact type grinders, such as a hammer mill, a pin mill and ajet mill, and mill type grinders, such as a cutter mill that performsgrinding by means of shear force produced by a rotary blade and an outerperipheral stator (stationary blade).

The temperature for carrying out the grinding treatment is preferably−200° C. or higher and lower than 50° C.

In one embodiment, the grinding is freeze grinding and can be usuallycarried out at a temperature in the range of −200° C. to −100° c. In thepresent embodiment, the temperature can be generally adjusted usingliquid nitrogen.

In one embodiment, the grinding is carried out at a temperature in thevicinity of room temperature (e.g., in the range of 10° C. to 50° C.).

In one embodiment, the grinding can be carried out at 10° C. or higherand lower than 50° C., is preferably carried out at a temperature in therange of 10° C. to 40° C., and is more preferably carried out at atemperature in the range of 10° C. to 30° C. The present embodiment ispreferable from the viewpoints of simplification of the step andreduction of cost required for the grinding.

The method for producing a composition containing low molecular weightPTFE of the present invention may further include classificationtreatment.

In the classification treatment, for example, airflow classification canbe used. Specifically, the classification treatment may be one in whichfine particles or fibrous particles are removed by airflowclassification and then coarse particles are further removed byclassification.

In the airflow classification, low molecular weight PTFE particles(e.g., ground particles) are fed to a column-like classification room bydecompressed air and dispersed by swirl airflow in the room, and thefine particles are classified by centrifugal force. The fine particlesare recovered with a cyclone and a bug filter from the center. In theclassification room, a cone in a conical shape or a rotating body suchas a rotor is installed in order to allow the ground particles and airto uniformly perform swirling motion.

When classification cones are used, adjustment of classification pointsis carried out by adjusting a flow rate of secondary air and a gapbetween the classification cones. When a rotor is used, a flow rate inthe classification room is adjusted by the number of revolutions of therotor.

Examples of the method for removing coarse particles include airflowclassification using a mesh, a vibrating screen and an ultrasonic sieve,and airflow classification is preferable.

In one embodiment, the classification treatment may be carried out priorto the step (I).

In one embodiment, the classification treatment is carried out on thecomposition having been subjected to the grinding treatment.

In a preferred embodiment, the step (II) is a step in which grindingtreatment is carried out on the composition containing low molecularweight PTFE obtained in the step (I), then classification treatment iscarried out, and thereafter, at least one selected from the groupconsisting of washing treatment, steam treatment and decompressiontreatment is carried out to obtain a composition containing lowmolecular weight PTFE.

In another preferred embodiment, the step (II) is a step in whichgrinding treatment is carried out on the composition containing lowmolecular weight PTFE obtained in the step (I), then at least oneselected from the group consisting of washing treatment, steam treatmentand decompression treatment is carried out, and thereafter,classification treatment is carried out to obtain a compositioncontaining low molecular weight PTFE.

A content of PFOA and its salt contained in the composition containinglow molecular weight PTFE obtained in the present invention ispreferably less than 25 ppb, more preferably 15 ppb or less, still morepreferably less than 5 ppb, in terms of mass, based on the low molecularweight PTFE. The lower limit of the content of PFOA and its salt is notlimited, but it is, for example, less than the detection limit, morespecifically 0 ppb.

According to the present invention, a composition containing lowmolecular weight PTFE and having a low content of PFOA and its salt asdescribed above can be obtained. The above content indicates a totalamount of PFOA and its salt. The salt of PFOA is as previouslydescribed.

The content of the PFOA and its salt can be measured by using liquidchromatography. More specifically, from the composition that is ameasuring object, the PFOA and its salt are extracted using a solvent,and the amount of the PFOA and its salt contained in the solvent afterextraction can be measured by using liquid chromatography.

A content of PFC contained in the composition containing low molecularweight PTFE obtained by the production method of the present inventionis preferably 50 ppb or less, more preferably less than 25 ppb, stillmore preferably 15 ppb or less, particularly preferably 5 ppb or less,very particularly preferably less than 5 ppb, in terms of mass, based onthe low molecular weight PTFE. The lower limit of the content of the PFCis not limited, but it is, for example, less than the detection limit,more specifically 0 ppb. According to the present invention, acomposition containing low molecular weight PTFE and having a lowcontent of PFC as described above can be obtained. When PFCs of aplurality of structures are present, the above content indicates a totalamount of PFCs of a plurality of structures. The PFC is as previouslydescribed.

The content of the PFC can be measured by using, for example, liquidchromatography. More specifically, from the composition that is ameasuring object, the PFC is extracted using a solvent, and the amountof the PFC contained in the solvent after extraction can be measured byusing liquid chromatography.

In one embodiment, the shape of the composition containing low molecularweight PTFE obtained by the production method of the present inventionis powder. A specific surface area of the powder of the presentembodiment is preferably in the range of 0.1 to 30.0 m²/g. The specificsurface area can be obtained by using a surface analyzer. Specifically,the specific surface area is measured by BET method using a mixed gasconsisting of 30% of nitrogen and 70% of helium as a carrier gas andusing liquid nitrogen as a refrigerant. The surface analyzer may be, forexample, BELSORP-mini II (trade name, manufactured by MicrotracBELCorp.). The specific surface area is more preferably 7.0 m²/g or more.

In one embodiment, the shape of the composition containing low molecularweight PTFE obtained by the production method of the present inventionis powder. A specific surface area of the powder of the presentembodiment is 0.1 m²/g or more and less than 7.0 m²/g. A method formeasuring the specific surface area is as described above. Such lowmolecular weight PTFE can be easily dispersed in a matrix material. Thematrix material may be a plastic, an ink, a coating material, or thelike.

The lower limit of the specific surface area of the powder(specifically, low molecular weight PTFE) of the present embodiment ispreferably 1.0 m²/g or more. The upper limit of the specific surfacearea of the powder of the present embodiment is preferably 5.0 m²/g orless, more preferably 3.0 m²/g or less.

In one embodiment, the shape of the composition containing low molecularweight PTFE obtained by the production method of the present inventionis powder. A specific surface area of the powder of the presentembodiment is 7.0 m²/g or more and 30.0 m²/g or less. A method formeasuring the specific surface area is as described above. In the caseof such low molecular weight PTFE, the sizes of particles dispersed in amatrix material are small, so that an effect of surface modification,such as an effect of enhancing texture of a coating surface, is high,and the oil absorption can be high. The matrix material may be an oil, agrease, a coating material, a plastic, or the like.

The lower limit of the specific surface area of the powder(specifically, low molecular weight PTFE) of the above embodiment ispreferably 8.0 m²/g or more. The upper limit of the specific surfacearea of the powder (specifically, low molecular weight PTFE contained inthe composition) of the present embodiment is preferably 30.0 m²/g orless, and is more preferably 20.0 m²/g or less.

In one embodiment, the shape of the composition containing low molecularweight PTFE obtained by the production method of the present inventionis powder. An average particle size of the powder of the presentembodiment is preferably in the range of 0.5 to 200 μm. The averageparticle size is more preferably 100 μm or less, still more preferably50 μm or less. The average particle size is, for example, 1.0 μm ormore. The low molecular weight PTFE obtained by the present inventionhas such an average particle size as above, and therefore, when it isadded to a coating material as, for example, an additive, it cancontribute to formation of a coating film having more excellent surfacesmoothness.

A method for measuring the average particle size of the compositioncontaining low molecular weight PTFE obtained by the production methodof the present invention is as described as the method for measuring theaverage particle size of the low molecular weight PTFE obtained in thestep (I).

In a preferred embodiment, the composition containing low molecularweight PTFE obtained by the production method of the present inventionis in powder form.

The composition containing low molecular weight PTFE obtained by thepresent invention can be used after it is molded, when needed.

In one embodiment, the method of the present invention is a method forpurifying a composition containing low molecular weight PTFE,comprising:

irradiating a composition containing PTFE with ionizing radiation toobtain a composition containing low molecular weight PTFE having a meltviscosity at 380° C. in the range of 1.0×10² to 7.0×10⁵ Pa·s; and

carrying out at least one selected from the group consisting of washingtreatment, steam treatment and decompression treatment on thecomposition containing low molecular weight PTFE obtained above.

The composition containing low molecular weight PTFE obtained by thepresent invention is preferably a powder, and can be preferably used asan additive for modifying a molding material, an ink, a cosmetic, acoating material, a grease, a member for office automation equipment, atoner or the like, an additive for a plating solution, or the like.Examples of the molding material include engineering plastics, such aspolyoxybenzoyl polyester, polyimide, polyamide, polyamideimide,polyacetal, polycarbonate and polyphenylene sulfide. The compositioncontaining low molecular weight PTFE can be preferably used particularlyas a thickening agent for grease.

The composition containing low molecular weight PTFE obtained by thepresent invention is preferably a powder, and can be used as an additivefor a molding material. The composition can be preferably used, forexample, for the purpose of enhancing non-stickiness and slidingproperty of a copy roll or a photoreceptor; for the purpose of enhancingtexture of engineering plastic molded products, such as a surface layersheet of furniture, a dashboard of an automobile, and a cover of anappliance; for the purpose of enhancing lubricity or abrasion resistanceof machine parts that can cause mechanical friction, such as alight-load bearing, a gear, a cum, buttons of push-button telephone, aprojector, camera parts, and a sliding material; or as a processing aidfor engineering plastics, or the like.

The composition containing low molecular weight PTFE obtained by thepresent invention is preferably a powder, and can be used for thepurpose of enhancing lubricity of a varnish or a paint, as an additivefor a coating material. The composition containing low molecular weightPTFE can be used for the purpose of enhancing lubricity of cosmeticssuch as foundation, as an additive for cosmetics.

The composition containing low molecular weight PTFE obtained by thepresent invention is preferably a powder, and can be preferably used forthe purpose of enhancing oil-repellency or water-repellency of a wax orthe like and for the purpose of enhancing lubricity of a thickeningagent for grease, a grease or a toner.

The composition containing low molecular weight PTFE obtained by thepresent invention is preferably a powder, and can be used also as anelectrode binder for a secondary battery or a fuel battery, a hardnessadjuster for an electrode binder, a water-repellent for an electrodesurface, or the like.

The composition containing low molecular weight PTFE obtained by thepresent invention is preferably a powder, and can be used together witha lubricating oil to prepare a grease. Since the grease is characterizedby containing the powdery composition containing low molecular weightPTFE and a lubricating oil, the composition containing low molecularweight PTFE can be uniformly and stably dispersed in the lubricatingoil. The grease can be excellent in properties such as heat resistance,load-bearing property, electrical insulation property and lowhygroscopicity.

The lubricating oil (base oil) may be a mineral oil, or may be asynthetic oil. Examples of the lubricating oil (base oil) includeparaffinic or naphthenic mineral oils, and synthetic oils, such assynthetic hydrocarbon oil, ester oil, fluorine oil and silicone oil.From the viewpoint of heat resistance, fluorine oil is preferable, andexamples of the fluorine oil include perfluoropolyether oil and a lowpolymer of ethylene chloride trifluoride. The low polymer of ethylenechloride trifluoride may have a weight-average molecular weight of 500to 1,200.

In the grease, other thickening agents may be further used incombination. Examples of the thickening agent include metallic soap,composite metallic soap, bentonite, phthalocyanine, silica gel, a ureacompound, a urea-urethane compound, a urethane compound, and an imidecompound. Examples of the metallic soap include sodium soap, calciumsoap, aluminum soap and lithium soap. Examples of the urea compound, theurea-urethane compound and the urethane compound include a diureacompound, a triurea compound, a tetraurea compound, other polyureacompounds, urea-urethane compounds and diurethane compounds, andmixtures thereof.

The grease preferably contains the powdery composition containing lowmolecular weight PTFE in an amount of 0.1 to 50 mass %, more preferablycontains the composition in an amount of 0.5 mass % or more, and morepreferably contains the composition in an amount of 30 mass % or less.Since the powdery composition containing low molecular weight PTFE iscontained in the above amount, a grease having an appropriate hardnesscan be obtained. The grease can exhibit sufficient lubricity and canexhibit appropriate sealing property.

The grease can further contain a solid lubricant, an extreme pressureagent, an antioxidant, an oiliness agent, a rust preventive, a viscosityindex improver, a detergent dispersant, etc.

EXAMPLES

The present invention will be more specifically described with referenceto the following Examples, but the present disclosure is in no waylimited to those Examples.

Comparative Example 1

In a barrier nylon bag, 50 g of POLYFLON® PTFE F-104 (manufactured byDAIKIN INDUSTRIES, LTD., concentration of PFC and concentration of PFOAwere each the detection limit or less) was weighed, and the bag wassealed by means of heat sealing. Subsequently, the PTFE F-104 in the bagwas irradiated with cobalt-60 γ-ray at 150 kGy at room temperature,thereby obtaining a low molecular weight PTFE powder. The resulting lowmolecular weight PTFE powder was used as a sample for measuring contentsof PFOA and a perfluorocarboxylic acid having 6 to 14 carbon atoms andits derivative. Further, a melt viscosity of the resulting low molecularweight PTFE powder was measured. The results are set forth in Table 2.

Examples 1

The low molecular weight PTFE powder obtained in Comparative Example 1was spread on a 200-mesh stainless steel sieve. Subsequently, using astainless steel pipe of ⅜ inch, steam of 100° C. was allowed to passfrom under the sieve for 30 minutes in such a manner that the steamuniformly came into contact with the low molecular weight PTFE. Thesteam-treated low molecular weight PTFE was used as a sample formeasurement of contents of PFOA, and a perfluorocarboxylic acid having 6to 14 carbon atoms and its derivative.

By drying the low molecular weight PTFE at 110° C. for 12 hours,drying-treated low molecular weight PTFE was obtained. A melt viscosityof the low molecular weight PTFE powder obtained after the drying wasmeasured. The results are shown in Table 2.

Examples 2 to 5

The low molecular weight PTFE powder obtained in Comparative Example 1was placed in a 6 L stainless steel container (with three-one motor,equipped with propeller stirring blade) and washed using a washingsolution. The washing-treated low molecular weight PTFE was used as asample for measurement of contents of PFOA, and a perfluorocarboxylicacid having 6 to 14 carbon atoms and its derivative. The type and thetemperature of the washing solution and the time in which washing wascarried out are as in Table 2.

By drying the low molecular weight PTFE at 110° C. for 12 hours,drying-treated low molecular weight PTFE was obtained. A melt viscosityof the low molecular weight PTFE powder obtained after the drying wasmeasured. The results are shown in Table 2.

Example 6

By allowing the low molecular weight PTFE powder obtained in ComparativeExample 1 to stand still at −0.09 MPa (gauge pressure) for 48 hours,treated low molecular weight PTFE was obtained. The resulting lowmolecular weight PTFE powder was used as a sample for measurement ofcontents of PFOA, and a perfluorocarboxylic acid having 6 to 14 carbonatoms and its derivative. Further, a melt viscosity of the resulting lowmolecular weight PTFE powder was measured. The results are shown inTable 2.

(Measurement of Melt Viscosity)

Melt viscosities of the low molecular weight PTFE obtained inComparative Example 1 and Example 6 and the drying-treated low molecularweight PTFE obtained in Examples 1 to 5 were each measured.

In accordance with ASTM D 1238, the melt viscosity was measured using 2g of a sample having been heated in advance at the measuring temperature(380° C.) for 5 minutes and using a flow tester (manufactured byShimadzu Corporation) and a die of 2ϕ-8L while maintaining the sample atthe above temperature under a load of 0.7 MPa.

(Measurement of Content of PFOA and its Salt)

Using a liquid chromatograph mass spectrometer (Waters, LC-MS ACQUITYUPLC/TQD), a content of PFOA contained in the sample for measurement wasmeasured.

Specifically, regarding the samples for measurement of ComparativeExample 1 and Example 6, 1 g of each sample was weighed (powder formeasurement), then 5 ml of acetonitrile was added, and the mixture wassubjected to ultrasonic treatment for 60 minutes to extract PFOA.Regarding the samples for measurement of Examples 1 to 5, 10 g of eachsample was weighed, and the sample was filtered using a 330-mesh JIStest sieve and subsequently subjected to tapping for 30 minutes toperform dehydration. To 1 g of the dehydrated powder (powder formeasurement), 5 ml of acetonitrile was added, and the mixture wassubjected to ultrasonic treatment for 60 minutes to extract PFOA. Awater content and a content of the low molecular weight PTFE in each ofthe powders for measurement of Examples 1 to 5 were determined by dryingthe powder for measurement in a dryer having been set to 120° C. andcarrying out calculation using the weights before and after the drying.

A content of PFOA in the resulting liquid phase was measured using MRM(Multiple Reaction Monitoring) method. Acetonitrile (A) and an ammoniumacetate aqueous solution (20 mmol/L) (B) were used as mobile phases, andthey were fed at a concentration gradient (A/B=40/60-2 min-80/20-1 min).ACQUITY UPLC BEH C18 1.7 μm was used as a separatory column, the columntemperature was set at 40° C., and the injection quantity was set to 5μL. In the ionization method, ESI (Electrospray ionization) Negative wasused, a cone voltage was set at 25 V, and measurement of precursor ionmolecular weight/product ion molecular weight resulted in 413/369. Thecontent of PFOA was calculated using an external standard method. Thedetection limit in this measurement was 5 ppb.

(Measurement of Content of Perfluorocarboxylic Acid Having 6 to 14Carbon Atoms and its Derivative)

Using a liquid chromatograph mass spectrometer (Waters, LC-MS ACQUITYUPLC/TQD), a content of a perfluorocarboxylic acid having 6 to 14 carbonatoms contained in the sample for measurement obtained in each ofExamples and Comparative Example was measured.

Specifically, the above content was measured by the use of the MRMmethod using, as a solution, a liquid phase extracted in the measurementof PFOA. Under the measurement conditions changed in concentrationgradient (A/B=10/90-1.5 min-90/10-3.5 min) from the measurementconditions for PFOA, the precursor ion molecular weight/product ionmolecular weight described in Table 1 were measured. The total amount ofperfluorocarboxylic acids having 6 to 14 carbon atoms was calculatedfrom the following formula using the content (X) of perfluorooctanoicacid obtained in the above measurement. The detection limit in thismeasurement was 5 ppb.(A_(c6)+A_(c7)+A_(c8)+A_(c9)+A_(c10)+A_(c11)+A_(c12)+A_(c13)+A_(c14))/A_(c8)×Xwherein A_(c6) to A_(c14) represent peak areas of carboxylic acidshaving 6 carbon atoms to 14 carbon atoms, respectively; and

X represents a content of perfluorooctanoic acid (8 carbon atoms)calculated from the result of the measurement by the MFM method, usingan external standard method.

TABLE 1 Number of carbon atoms Precursor ion Product ion of carboxylicacid molecular weight molecular weight 6 313 269 7 363 319 8 413 369 9463 419 10 513 469 11 563 519 12 613 569 13 663 619 14 713 669

The results are shown in the following table. In the following table,the “content of C6 to 14” indicates a content of perfluorocarboxylicacids having 6 to 14 carbon atoms and their derivatives. In thefollowing table, the content of PFOA and its salt in each of Examples 1to 5 is a content of PFOA and its salt based on 100 mass parts of thelow molecular weight PTFE after drying at 120° C.; and the content of C6to 14 in each of Examples 1 to 5 is a content of C6 to 14 based on 100mass parts of the low molecular weight PTFE after drying at 120° C. Thecontent of PFOA and its salt in each of Comparative Example 1 andExample 6 is a content of PFOA and its salt based on 100 mass parts ofthe low molecular weight PTFE obtained in each of Comparative Example 1and Example 6; and the content of C6 to 14 in each of ComparativeExample 1 and Example 6 is a content of C6 to 14 based on 100 mass partsof the low molecular weight PTFE obtained in each of Comparative Example1 and Example 6.

[Table 2]

Step (II) 10% NaOH aqueous Raw material solution washing PTFE treatmentmanufactured Stream passing Ethanol washing Temperature by Daikin Step(I) treatment treatment of NaOH Industries Irradition with ionizingradiation Steam Passing Temperature Washing aqueous Washing Ltd.Temperature Atmosphere Dose temperature time of ethanol time solutiontime Comp. F-104 room air 150 Ex. 1 temperature kGy Ex. 1 F-104 room air150 100° C. 30 temperature kGy minutes Ex. 2 F-104 room air 150 25° C.30 temperature kGy minutes Ex. 3 F-104 room air 150 25° C. 60temperature kGy minutes Ex. 4 F-104 room air 150 temperature kGy Ex. 5F-104 room air 150 95° C. 10 temperature kGy minutes Ex. 6 F-104 roomair 150 temperature kGy Raw material PTFE Step (II) manufactured Acetonewashing Decompression Content Content by Daikin treatment treatmentDrying treatment of of C6 Melt Industries Temperature WashingDecompression Drying Drying PFOA to 14 viscosity Ltd. of acetone timePressure time temperature time (ppb) (ppb) (Pa · s) Comp. F-104 92 2555.5 × 10⁴ Ex. 1 Ex. 1 F-104 110° C. 12 <5 <5 5.6 × 10⁴ hours Ex. 2 F-104110° C. 12 <5 <5 5.6 × 10⁴ hours Ex. 3 F-104 110° C. 12 <5 <5 5.6 × 10⁴hours Ex. 4 F-104 25° C. 30 110° C. 12 <5 <5 5.6 × 10⁴ minutes hours Ex.5 F-104 110° C. 12 <5 <5 5.6 × 10⁴ hours Ex. 6 F-104 −0.09 48 hours 1824 5.6 × 10⁴ MPa (gauge pressure)

INDUSTRIAL APPLICABILITY

The low molecular weight PTFE obtained in the production method of thepresent invention can be usefully used for producing plastics, inks,cosmetics, coating materials, greases, etc.

The present invention includes following embodiments:

Embodiment 1

A method for producing a composition containing low molecular weightpolytetrafluoroethylene, comprising:

(I) irradiating a composition containing high molecular weightpolytetrafluoroethylene with ionizing radiation to obtain a compositioncontaining low molecular weight polytetrafluoroethylene having a meltviscosity at 380° C. in the range of 1.0×10² to 7.0×10⁵ Pa·s; and

(II) carrying out at least one selected from the group consisting ofwashing treatment, steam treatment and decompression treatment on thecomposition containing low molecular weight polytetrafluoroethylene.

Embodiment 2

The method for producing a composition containing low molecular weightpolytetrafluoroethylene according to Embodiment 1, wherein the step (II)comprises carrying out at least one selected from the group consistingof washing treatment, steam treatment and decompression treatment on thecomposition containing low molecular weight polytetrafluoroethyleneobtained in the step (I) and carrying out grinding treatment.

Embodiment 3

The method for producing a composition containing low molecular weightpolytetrafluoroethylene according to Embodiment 1 or 2, wherein the step(II) comprises carrying out grinding treatment on the compositioncontaining low molecular weight polytetrafluoroethylene obtained in thestep (I) and thereafter carrying out at least one selected from thegroup consisting of washing treatment, steam treatment and decompressiontreatment.

Embodiment 4

The method for producing a composition containing low molecular weightpolytetrafluoroethylene according to any one of Embodiments 1 to 3,wherein washing using a solution containing at least one selected fromthe group consisting of water and an organic solvent is carried out asthe washing treatment.

Embodiment 5

The method for producing a composition containing low molecular weightpolytetrafluoroethylene according to any one of Embodiments 1 to 4,wherein the steam treatment uses water, an alkaline aqueous solution ora polar solvent as a treatment solution in the form of steam at atemperature in the range of 90° C. to 160° C.

Embodiment 6

The method for producing a composition containing low molecular weightpolytetrafluoroethylene according to any one of Embodiments 1 to 5,wherein the composition containing high molecular weightpolytetrafluoroethylene is powder.

Embodiment 7

The method for producing a composition containing low molecular weightpolytetrafluoroethylene according to any one of Embodiments 1 to 6,wherein the composition containing low molecular weightpolytetrafluoroethylene obtained in the step (I) further contains a lowmolecular weight fluorine-containing compound; and

the low molecular weight fluorine-containing compound is an acid havinga fluorine-containing carbon chain having 6 to 14 carbon atoms or aderivative thereof.

Embodiment 8

The method for producing a composition containing low molecular weightpolytetrafluoroethylene according to Embodiment 7, wherein a content ofthe low molecular weight fluorine-containing compound in the compositioncontaining low molecular weight polytetrafluoroethylene obtained in thestep (II) is 50 mass ppb or less based on the low molecular weightpolytetrafluoroethylene.

Embodiment 9

The method for producing a composition containing low molecular weightpolytetrafluoroethylene according to any one of Embodiments 1 to 8,wherein the low molecular weight fluorine-containing compound in thecomposition containing low molecular weight polytetrafluoroethyleneobtained in the step (II) contains perfluorooctanoic acid and a saltthereof, and

a content of the perfluorooctanoic acid and a salt thereof is less than25 mass ppb based on the low molecular weight polytetrafluoroethylene.

The invention claimed is:
 1. A method for producing a compositioncontaining low molecular weight polytetrafluoroethylene, comprising: (I)irradiating a composition containing high molecular weightpolytetrafluoroethylene with ionizing radiation to obtain a compositioncontaining low molecular weight polytetrafluoroethylene having a meltviscosity at 380° C. in the range of 1.0×10² to 7.0×10⁵ Pa·s; and (II)after the irradiation, carrying out steam treatment on the compositioncontaining low molecular weight polytetrafluoroethylene, and the steamtreatment uses water, an alkaline aqueous solution or a polar solvent asa treatment solution in the form of steam at a temperature in the rangeof 90° C. to 160° C.
 2. The method for producing a compositioncontaining low molecular weight polytetrafluoroethylene according toclaim 1, wherein the step (II) comprises carrying out grinding treatmenton the composition containing low molecular weightpolytetrafluoroethylene obtained in the step (I) and thereafter carryingout steam treatment.
 3. The method for producing a compositioncontaining low molecular weight polytetrafluoroethylene according toclaim 1, wherein the composition containing high molecular weightpolytetrafluoroethylene is powder.
 4. The method for producing acomposition containing low molecular weight polytetrafluoroethyleneaccording to claim 1, wherein the composition containing low molecularweight polytetrafluoroethylene obtained in the step (I) further containsa low molecular weight fluorine-containing compound; and the lowmolecular weight fluorine-containing compound is an acid having afluorine-containing carbon chain having 6 to 14 carbon atoms or aderivative thereof.
 5. The method for producing a composition containinglow molecular weight polytetrafluoroethylene according to claim 4,wherein a content of the low molecular weight fluorine-containingcompound in the composition containing low molecular weightpolytetrafluoroethylene obtained in the step (II) is 50 mass ppb or lessbased on the low molecular weight polytetrafluoroethylene.
 6. The methodfor producing a composition containing low molecular weightpolytetrafluoroethylene according to claim 1, wherein the low molecularweight fluorine-containing compound in the composition containing lowmolecular weight polytetrafluoroethylene obtained in the step (II)contains perfluorooctanoic acid and a salt thereof; and a content of theperfluorooctanoic acid and a salt thereof is less than 25 mass ppb basedon the low molecular weight polytetrafluoroethylene.